In an information state where various agents have both factual knowledge and knowledge about each other, announcements can be made that change the state of information. Such informative announcements can have the curious property that they become false because they are announced. The most typical example of that is 'fact p is true and you don't know that', after which you know that p, which entails the negation of the announcement formula. The announcement of such a formula in a given (...) information state is called an unsuccessful update. A successful formula is a formula that always becomes common knowledge after being announced. Analysis of information systems and 'philosophical puzzles' reveals a growing number of dynamic phenomena that can be described or explained by unsuccessful updates. This increases our understanding of such philosophical problems. We also investigate the syntactic characterization of the successful formulas. (shrink)

Can victims of the oracle paradox, which is known primarily through its unexpected hanging and surprise examination versions, extricate themselves from their difficulties of reasoning? No. For they do not, contrary to recent opinion, commit errors of fallacious elimination. As I shall argue, the difficulties of reasoning faced by these victims do not originate in the domain of concepts, propositions and their entailment relations; nor do they result from misapprehensions about limitations on what can be known. The difficulties of reasoning (...) flow, instead, from conflicts that arise in the practical dimension of life. The oracle paradox is in this way more evocative of problems faced in the theory of computation than it is like the celebrated Russell’s paradox. (shrink)